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Register a new userExploring Reionization-Era Quasars IV: Discovery of Six New $z gtrsim 6.5$ Quasars with DES, VHS and unWISE Photometry
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This is the fourth paper in a series of publications aiming at discovering quasars at the epoch of reionization. In this paper, we expand our search for $zsim 7$ quasars to the footprint of the Dark Energy Survey (DES) Data Release One (DR1), covering $sim 5000$ deg$^2$ of new area. We select $zsim 7$ quasar candidates using deep optical, near-infrared (near-IR) and mid-IR photometric data from the DES DR1, the VISTA Hemisphere Survey (VHS), the VISTA Kilo-degree Infrared Galaxy (VIKING) survey, the UKIRT InfraRed Deep Sky Surveys -- Large Area Survey (ULAS) and the unblurred coadds from the Wide-field Infrared Survey Explore ($WISE$) images (unWISE). The inclusion of DES and unWISE photometry allows the search to reach $sim$ 1 magnitude fainter, comparing to our $z gtrsim 6.5$ quasar survey in the northern sky (Wang et al. 2018). We report the initial discovery and spectroscopic confirmation of six new luminous quasars at $z>6.4$, including an object at $z=7.02$, the fourth quasar yet known at $z>7$, from a small fraction of candidates observed thus far. Based on the recent measurement of $z sim 6.7 $ quasar luminosity function using the quasar sample from our survey in the northern sky, we estimate that there will be $gtrsim$ 55 quasars at $z > 6.5$ at $M_{1450} < -24.5$ in the full DES footprint.
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We report the results from a search for z > 6.5 quasars using the Dark Energy Survey (DES) Year 3 dataset combined with the VISTA Hemisphere Survey (VHS) and WISE All-Sky Survey. Our photometric selection method is shown to be highly efficient in identifying clean samples of high-redshift quasars leading to spectroscopic confirmation of three new quasars - VDESJ 0244-5008 (z=6.724), VDESJ 0020-3653 (z=6.834) and VDESJ 0246-5219 (z=6.90) - which were selected as the highest priority candidates in the survey data without any need for additional follow-up observations. The new quasars span the full range in luminosity covered by other z>6.5 quasar samples (J AB = 20.2 to 21.3; M1450 = -25.6 to -26.6). We have obtained spectroscopic observations in the near infrared for VDESJ 0244-5008 and VDESJ 0020-3653 as well as our previously identified quasar, VDESJ 0224-4711 at z=6.50 from Reed et al. (2017). We use the near infrared spectra to derive virial black-hole masses from the full-width-half-maximum of the MgII line. These black-hole masses are ~ 1 - 2 x 10$^9$M$_odot$. Combining with the bolometric luminosities of these quasars of L$_{rm{bol}}simeq$ 1 - 3 x 10$^{47}$implies that the Eddington ratios are high - $simeq$0.6-1.1. We consider the Ctextrm{textsc{IV}} emission line properties of the sample and demonstrate that our high-redshift quasars do not have unusual Ctextrm{textsc{IV}} line properties when compared to carefully matched low-redshift samples. Our new DES+VHS $z>6.5$ quasars now add to the growing census of luminous, rapidly accreting supermassive black-holes seen well into the epoch of reionisation.
This is the third paper in a series aims at finding reionzation-era quasars with the combination of DESI Legacy imaging Surveys (DELS) and near-infrared imaging surveys, such as the UKIRT Hemisphere Survey (UHS), as well as the Wide-field Infrared Survey Explore ($WISE$) mid-infrared survey. In this paper, we describe the updated quasar candidate selection procedure, report the discovery of 16 quasars at $6.4lesssim z lesssim6.9$ from area of $sim$13,020 deg$^2$, and present the quasar luminosity function (QLF) at $zsim6.7$. The measured QLF follows $Phi(L_{1450})propto L_{1450}^{-2.35}$ in the magnitude range $27.6<M_{1450}<-25.5$. We determine the quasar comoving spatial density at $langle z rangle$=6.7 and $M_{1450}<-26.0$ to be $rm 0.39pm0.11 Gpc^{-3}$ and find that the exponential density evolution parameter to be $k=-0.78pm0.18$ from $zsim6$ to $zsim6.7$, corresponding to a rapid decline by a factor of $sim 6$ per unit redshift towards earlier epoch, a rate significantly faster than that at $zsim 3- 5$. The cosmic time between $zsim6$ and $zsim6.7$ is only 121 Myrs. The quasar density declined by a factor of more than three within such short time requires that SMBHs must grow rapidly or they are less radiatively efficient at higher redshifts. We measured quasar comoving emissivity at $zsim6.7$ which indicate that high redshift quasars are highly unlikely to make a significant contribution to hydrogen reionization. The broad absorption line (BAL) quasar fraction at $zgtrsim6.5$ is measured to be $gtrsim$22%. In addition, we also report the discovery of additional five quasars at $zsim6$ in the appendix.
We report discovery of 41 new high-z quasars and luminous galaxies, which were spectroscopically identified at 5.7 < z < 6.9. This is the fourth in a series of papers from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, based on the deep multi-band imaging data collected by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. We selected the photometric candidates by a Bayesian probabilistic algorithm, and then carried out follow-up spectroscopy with the Gran Telescopio Canarias and the Subaru Telescope. Combined with the sample presented in the previous papers, we have now spectroscopically identified 137 extremely-red HSC sources over about 650 deg2, which include 64 high-z quasars, 24 high-z luminous galaxies, 6 [O III] emitters at z ~ 0.8, and 43 Galactic cool dwarfs (low-mass stars and brown dwarfs). The new quasars span the luminosity range from M1450 ~ -26 to -22 mag, and continue to populate a few magnitude lower luminosities than have been probed by previous wide-field surveys. In a companion paper, we derive the quasar luminosity function at z ~ 6 over an unprecedentedly wide range of M1450 ~ -28 to -21 mag, exploiting the SHELLQs and other survey outcomes.
Luminous distant quasars are unique probes of the high redshift intergalactic medium (IGM) and of the growth of massive galaxies and black holes in the early universe. Absorption due to neutral Hydrogen in the IGM makes quasars beyond a redshift of z~6.5 very faint in the optical $z$-band, thus locating quasars at higher redshifts require large surveys that are sensitive above 1 micron. We report the discovery of three new z>6.5 quasars, corresponding to an age of the universe of <850 Myr, selected as z-band dropouts in the Pan-STARRS1 survey. This increases the number of known z>6.5 quasars from 4 to 7. The quasars have redshifts of z=6.50, 6.52, and 6.66, and include the brightest z-dropout quasar reported to date, PSO J036.5078+03.0498 with M_1450=-27.4. We obtained near-infrared spectroscopy for the quasars and from the MgII line we estimate that the central black holes have masses between 5x10^8 and 4x10^9 M_sun, and are accreting close to the Eddington limit (L_Bol/L_Edd=0.13-1.2). We investigate the ionized regions around the quasars and find near zone radii of R_NZ=1.5-5.2 proper Mpc, confirming the trend of decreasing near zone sizes with increasing redshift found for quasars at 5.7<z<6.4. By combining R_NZ of the PS1 quasars with those of 5.7<z<7.1 quasars in the literature, we derive a luminosity corrected redshift evolution of R_NZ,corrected=(7.2+/-0.2)-(6.1+/-0.7)x(z-6) Mpc. However, the large spread in R_NZ in the new quasars implies a wide range in quasar ages and/or a large variation in the neutral Hydrogen fraction along different lines of sight.
Quasars are galaxies hosting accreting supermassive black holes; due to their brightness, they are unique probes of the early universe. To date, only few quasars have been reported at $z > 6.5$ ($<$800 Myr after the Big Bang). In this work, we present six additional $z gtrsim 6.5$ quasars discovered using the Pan-STARRS1 survey. We use a sample of 15 $z gtrsim 6.5$ quasars to perform a homogeneous and comprehensive analysis of this highest-redshift quasar population. We report four main results: (1) the majority of $zgtrsim$6.5 quasars show large blueshifts of the broad CIV 1549AA$,$emission line compared to the systemic redshift of the quasars, with a median value $sim$3$times$ higher than a quasar sample at $zsim$1; (2) we estimate the quasars black hole masses (M$rm_{BH}sim$0.3$-$5 $times$ 10$^{9}$ M$_{odot}$) via modeling of the MgII 2798AA$,$emission line and rest-frame UV continuum; we find that quasars at high redshift accrete their material (with $langle (L_{mathrm{bol}}/L_{mathrm{Edd}}) rangle = 0.39$) at a rate comparable to a luminosity-matched sample at lower$-$redshift, albeit with significant scatter ($0.4$ dex); (3) we recover no evolution of the FeII/MgII abundance ratio with cosmic time; (4) we derive near zone sizes; together with measurements for $zsim6$ quasars from recent work, we confirm a shallow evolution of the decreasing quasar near zone sizes with redshift. Finally, we present new millimeter observations of the [CII] 158 $mu$m emission line and underlying dust continuum from NOEMA for four quasars, and provide new accurate redshifts and [CII]/infrared luminosities estimates. The analysis presented here shows the large range of properties of the most distant quasars.
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